Volcanoes metal complexes

The composition of the particles is related to that of the source rocks. Quartz sand [composed of silica (silicon dioxide)], which makes up the most common variety of silica sand, is derived from quartz rocks. Pure quartz is usually almost free of impurities and therefore almost colorless (white). The coloration of some silica sand is due to chemical impurities within the structure of the quartz. The common buff, brown, or gray, for example, is caused by small amounts of metallic oxides iron oxide makes the sand buff or brown, whereas manganese dioxide makes it gray. Other minerals that often also occur as sand are calcite, feldspar and obsidian Calcite (composed of calcium carbonate), is generally derived from weathered limestone or broken shells or coral feldspar is an igneous rock of complex composition, and obsidian is a natural glass derived from the lava erupting from volcanoes see Chapter 2. 

No catalyst has an infinite lifetime. The accepted view of a catalytic cycle is that it proceeds via a series of reactive species, be they transient transition state type structures or relatively more stable intermediates. Reaction of such intermediates with either excess ligand or substrate can give rise to very stable complexes that are kinetically incompetent of sustaining catalysis. The textbook example of this is triphenylphosphine modified rhodium hydroformylation, where a plot of activity versus ligand metal ratio shows the classical volcano plot whereby activity reaches a peak at a certain ratio but then falls off rapidly in the presence of excess phosphine, see Figure... 

Many other metals have been shown to be active in HDS catalysis, and a number of papers have been published on the study of periodic trends in activities for transition metal sulfides [15, 37-43]. Both pure metal sulfides and supported metal sulfides have been considered and experimental studies indicate that the HDS activities for the desulfurization of dibenzothiophene [37] or of thiophene [38, 39] are related to the position of the metal in the periodic table, as exemplified in Fig. 1.2 (a), 1.2 (b), and 1.2 (c). Although minor differences can be observed from one study to another, all of them agree in that second and third row metals display a characteristic volcano-type dependence of the activity on the periodic position, and they are considerably more active than their first row counterparts. Maximum activities were invariably found around Ru, Os, Rh, Ir, and this will be important when considering organometallic chemistry related to HDS, since a good proportion of that work has been concerned with Ru, Rh, and Ir complexes, which are therefore reasonable models in this sense however, Pt and Ni complexes have also been recently shown to promote the very mild stoichiometric activation and desulfurization of substituted dibenzothiophenes (See Chapter 4). 

Recently there have appeared papers by other authors in which volcano-shaped curves have been obtained. Fahrenfort, van Reijen, and Sachtler (467) have carried out complex kinetic, IR spectroscopic, calorimetric, and mass spectrometric investigations on the decomposition of formic acid on various metals. The authors come to the conclusion that the reaction proceeds via the intermediate formation of an adsorption complex of the surface nickel formate type. By comparing the heat of formation of the formate of the corresponding metal with the temperature Tr at which a fixed depth of conversion r (log r = —0.8) is reached, the authors have obtained a broken line similar to the Balandin volcano-shaped curves (Fig. 63). The catalyst half-covered with the adsorption complex is the most active one. The reaction investigated by the authors differs from those investigated by us. It is characteristic, however, that in the case of oxides the selectivity is the same with respect to the decomposition of alcohols and of formic acid [Fig. 1 in Mars (468)). In their report at the Paris Congress on Catalysis Sachtler and Fahrenfort (469) give additional data on volcano-shaped curves for a number of reactions and point out that this relationship between the catalytic activity and the stability of the intermediate complex has been qualitatively predicted by Balandin. ... 

If one moves in the periodic table or in the volcano plot of Figure 2.13 in the direction of metals that have more electrons in the d-orbitals, the activity decreases Ni, Cu, and Zn macrocyclics have low activity and catalyze the reduction of O2 only to peroxide. The lowest activity is shown by CuPc and CuTSPc. Ni, Cu, and Zn complexes do not exhibit the M(IIF(II) transition because between the energy levels of the HOMO and the LUMO of the ligand, they have no energy levels with d-character like for example Cr, Mn, Fe, and Co complexes "-So the low activity is associated to the fact that for these complexes the frontier orbitals have no d-character, so O2 cannot bind to the metal center or the interaction is not favorable. This is clearly illustrated for CuPc in Figures 2.14 and 2.15. Their catalytic activity is then always lower than that found for iron and cobalt chelates Pt, Rh, and Os macrocyclics show some activity but they give peroxide as the main product of the reaction. ... 

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